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| United States Patent |
5,146,391
|
|
MacFarlane
, et al.
|
September 8, 1992
|
Crosslinked electrolyte capacitors and methods of making the same
Abstract
Compact leak-proof electrolytic capacitors including, between the anode and
the cathode, an ultrathin layer of a solid electrolyte, are disclosed. The
solid electrolyte comprises a solid solution of (a) an alkali metal salt,
a transition metal salt, an ammonium salt, an organic ammonium salt, a
zinc salt, a cadmium salt, a mercury salt or a thallium salt of (b) a
monbasic, dibasic or tribasic acid other than a haloid acid (c) in a
polymer of high solvation power. Preferred salts are the
tetrafluoroborates and hexafluoroglutarates of sodium and potassium, and
the preferred polymer is a blend of polyethylene oxide with a
siloxane-alkylene oxide copolymer. Crosslinking of the polymer is
accomplished by using an agent which may be a di -, tri, or
polyisocyanate, a multifunctional reagent which is an analogue of the
compound to be crosslinked or di - and multifunctional acids, or di - and
multifunctional amines. Methods of making such capacitors are also
disclosed. Rolled solid electrolyte capacitors of this type are
characterized by low volume, absence of electrolyte leakage, and minimum
dielectric deformation, and are capable of delivering intense bursts of
current on demand, thereby being suitable for use in biomedical electronic
devices such as cardiac pacemakers and defibrillators implanted in the
human body.
| Inventors:
|
MacFarlane; Douglas R. (Elsternwick, AU);
Philpott; Arthur K. (Neerim South, AU);
Tetaz; John R. (Templestowe, AU)
|
| Assignee:
|
Specialised Conductives Pty. Ltd. (Neerim South, Victoria, AU)
|
| Appl. No.:
|
431600 |
| Filed:
|
November 3, 1989 |
| Current U.S. Class: |
361/525; 29/25.03; 252/578 |
| Intern'l Class: |
H01G 003/06; C08G 065/32; H01B 007/18 |
| Field of Search: |
361/323,311-315,502,523-527
29/25.42,25.03
252/62.2,500,63.2
429/213,194
128/419 P,419 PG,419 PT
428/409,421
|
References Cited
U.S. Patent Documents
| 3331002 | Jul., 1967 | Everitt | 361/527.
|
| 3869652 | Mar., 1975 | Maillot | 361/323.
|
| 3883784 | May., 1975 | Peck et al. | 361/525.
|
| 3988496 | Oct., 1976 | Biggs et al. | 428/383.
|
| 4092452 | May., 1978 | Hori et al. | 428/215.
|
| 4200701 | Apr., 1980 | Wetton et al. | 252/63.
|
| 4306273 | Dec., 1981 | Maylanot et al. | 361/318.
|
| 4723347 | Feb., 1988 | Burzi et al. | 29/25.
|
| 4730239 | Mar., 1988 | Currie et al. | 361/502.
|
| 4740869 | Apr., 1988 | Toshiya et al. | 361/505.
|
| 4787010 | Nov., 1988 | Bentley | 361/323.
|
| 4942501 | Jul., 1990 | MacFarlane et al. | 361/525.
|
Primary Examiner: Griffin; Donald
Attorney, Agent or Firm: Gottlieb, Rackman & Reisman
Parent Case Text
This application is a continuation-in-part of application Ser. No. 187,239
filed Apr. 29, 1988 now U.S. Pat. No. 4,942,501 issued Jul. 17, 1990.
Claims
We claim:
1. A crosslinked polymer electrolyte having a composition comprising (a) an
amorphous solid solution of (i) at least one salt selected from the group
consisting of alkali metal salts, alkali earth metal salts, transition
metal salts, ammonium salts, organic ammonium salts, zinc salts, cadmium
salts, mercury salts and thallium salts of (ii) at least one acid selected
from the group consisting of monobasic, dibasic and tribasic acids other
than haloid acids (iii) in a polymer, and (b) a crosslinking agent which
crosslinks the polymer.
2. The electrolyte of claim 1, wherein said polymer includes at least one
of mono-, di-, tri- and poly-functional compounds having alkyleneoxide
repeat units.
3. The electrolyte of claim 1, wherein the polymer includes at least one
hydroxy compound.
4. The electrolyte of claim 3, wherein said at least one hydroxy compound
includes:
##STR3##
where R and R' are H or an alkyl group.
5. The electrolyte of claim 3, wherein the molecular weight of said at
least one hydroxy compound is sufficiently low so that the polymer is
crosslinked without crystallizing.
6. The electrolyte of claim 1, wherein the crosslinking agent is a
diisocyanate.
7. The electrolyte of claim 6, wherein the diisocyanate is selected from
the group consisting of hexamethylene diisocyanate, toluenediisocyanate
and methyl diphenylisocyanate.
8. The electrolyte of claim 1, wherein each said at least one compound is
terminated with an unsaturated functional group.
9. The electrolyte of claim 8, wherein said unsaturated functional group is
selected from the group consisting of vinyl, allyl, acrylic and
alkacrylic.
10. The electrolyte of claim 9, wherein said crosslinking agent is
poly(alkylhydrogensiloxane).
11. The electrolyte of claim 9, wherein said crosslinking agent is a
difunctional reagent.
12. The electrolyte of claim 11, wherein said difunctional reagent is an
analog of the compound to be crosslinked.
13. The electrolyte of claim 11, wherein said difunctional reagent is
selected from the group consisting of a divinyl, diallyl, dialkacrylic and
diacrylic analog of the compound to be crosslinked.
14. The electrolyte of claim 8, wherein said compound is selected from the
group consisting of:
CXY.dbd.CRCOO--(--CH.sub.2 CHR'O--)--R"
CXY.dbd.CRCH.sub.2 O(--CH.sub.2 CHR'O--)--R"
where X,Y,R and R" are H or alkyl groups and R" is H, an alkyl group or,
for a difunctional compound, an unsaturated functional group.
15. The electrolyte of claim 1, wherein said crosslinking agent is selected
from the group consisting of di- and multifunctional acids.
16. The electrolyte of claim 1, wherein said crosslinking agent is selected
from the group consisting of di- and multifunctional amines
17. The electrolyte of claim 1, wherein said at least one salt is
NaBF.sub.4.
18. The electrolyte of claim 1, wherein said at least one salt is KSCN.
19. The electrolyte of claim 1, further comprising a plasticizer having a
chemical compatibility with the polymer and in which said at least one
salt is soluble.
20. The electrolyte of claim 19, wherein said plasticizer is one of low
molecular weight alkylene oxide oligomers and polymers having minimal
reaction with said polymer.
21. The electrolyte of claim 20, wherein said plasticizer contains at least
one end group comprised of one of an alkyl ether, an alkyl urethane and an
ester.
22. The electrolyte of claim 1, wherein the polymer is ethylene oxide based
and includes pendant groups for disrupting crystallization.
23. The electrolyte of claim 22, wherein the polymer is selected from the
group consisting of copolymers of ethylene oxide and propylene oxide.
24. The electrolyte of claim 23, wherein between substantially six and one
hundred percent of repeat units in the polymer ar propylene oxide groups.
25. The electrolyte of any one of claims 1 to 24, formed as a layer in
combination with an anode disposed to contact a first side of said layer
and a cathode disposed to contact a second side of said layer opposite
said first side.
26. A compact capacitor including an electrically conductive anode, an
electrically conductive cathode and between said anode and said cathode a
crosslinked polymer electrolyte having a composition comprising (a) an
amorphous solid solution of (i) at least one salt selected from the group
consisting of alkali metal salts, alkali earth metal salts, transition
metal salts, ammonium salts, organic ammonium salts, zinc salts, cadmium
salts, mercury salts and thallium salts of (ii) at least one acid selected
from the group consisting of monobasic, dibasic and tribasic acids other
than haloid acids (iii) in a polymer, and (b) a crosslinking agent which
crosslinks the polymer.
27. The capacitor of claim 26, wherein said polymer includes at least one
of mono-, di-, tri- and polyfunctional compounds having alkyleneoxide
repeat units.
28. The capacitor of claim 26, wherein the polymer includes at least one
hydroxy compound.
29. The capacitor of claim 28, wherein said at least one hydroxy compound
includes:
##STR4##
where R and R' are H or an alkyl group.
30. The capacitor of claim 28, wherein the molecular weight of said at
least one hydroxy compound is sufficiently low so that the polymer is
crosslinked without crystallizing.
31. The capacitor of claim 26, wherein the crosslinking agent is a
diisocyanate.
32. The capacitor of claim 31, wherein the diisocyanate is selected from
the group consisting of hexamethylene diisocyanate, toluenediisocyanate
and methyl diphenylisocyanate.
33. The capacitor of claim 26, wherein each said at least one compound is
terminated with an unsaturated functional group.
34. The capacitor of claim 33, wherein said unsaturated functional group is
selected from the group consisting of vinyl, allyl, acrylic and
alkacrylic.
35. The capacitor of claim 34, wherein said crosslinking agent is
poly(alkylhydrogensiloxane).
36. The capacitor of claim 34, wherein said crosslinking agent is a
difunctional reagent.
37. The capacitor of claim 36, wherein said difunctional reagent is an
analog of the compound to be crosslinked.
38. The capacitor of claim 36, wherein said difunctional reagent is
selected from the group consisting of a divinyl, diallyl, dialkacrylic and
diacrylic analog of the compound to be crosslinked.
39. The capacitor of claim 33, wherein said compound is selected from the
group consisting of:
CXY.dbd.CRCOO--(--CH.sub.2 CHR'O--)--R"
CXY.dbd.CRCH.sub.2 O(--CH.sub.2 CHR'O--)--R"
where X,Y,R and R' are H or alkyl groups and R" is H, an alkyl group or,
for a difunctional compound, an unsaturated functional group.
40. The capacitor of claim 26, wherein said crosslinking agent is selected
from the group consisting of di- and multifunctional acids.
41. The capacitor of claim 26, wherein said crosslinking agent is selected
from the group consisting of di- and multifunctional amines.
42. The capacitor of claim 26, wherein said at least one salt is
NaBF.sub.4.
43. The capacitor of claim 26, wherein said at least one salt is KSCN.
44. The capacitor of claim 26, further comprising a plasticizer having a
chemical compatibility with the polymer and in which said at least one
salt is soluble.
45. The capacitor of claim 44, wherein said plasticizer is one of low
molecular weight alkylene oxide oligomers and polymers having minimal
reaction with said polymer.
46. The capacitor of claim 45, wherein said plasticizer contains at least
one end group comprised of one of an alkyl ether, an alkyl urethane and an
ester.
47. The capacitor of claim 26, wherein the polymer is ethylene oxide based
and includes pendant groups for disrupting crystallization.
48. The capacitor of claim 47, wherein the polymer is selected from the
group consisting of copolymers of ethylene oxide and propylene oxide.
49. The capacitor of claim 48, wherein between substantially six and one
hundred percent of repeat units in the polymer are propylene oxide groups.
50. A method for forming an electrolytic capacitor comprising the steps of:
coating a cathode foil with a first coating of a polymerizable material
having a crosslinking agent dissolved therein;
curing the first coating;
coating an anode foil with a second coating of a polymerizable material
having a salt and a crosslinking agent dissolved therein;
assembling the anode and the cathode to one another in facing relationship
to form a capacitor assembly;
curing the second coating.
51. The method of claim 50, wherein the cathode foil has larger dimensions
than the anode foil, further comprising the step of enfolding the anode
foil within the cathode foil.
52. The method of claim 51, further comprising the step of expelling excess
anode coating material by applying pressure.
53. The method of claim 52, wherein pressure is applied by rolling.
54. The method of claim 51, further comprising the step of dissolving a
salt in the first coating material before coating the cathode.
55. The method of claim 54, further comprising the step of winding the
assembly into a roll.
56. The method of claim 50, further comprising the step of winding the
assembly into a roll.
57. The method of claim 50, wherein the cathode foil is a continuous ribbon
and the step of curing the first coating is achieved by passing the ribbon
through a curing oven.
58. The method of claim 50, wherein the anode foil is a continuous ribbon
and the second coating is applied by passing the ribbon through a bath of
the polymerizable material.
59. The method of claim 50, wherein the step of assembling the anode and
the cathode comprises cowinding the coated anode foil with a ribbon of
coated cathode foil.
60. The method of claim 59, further comprising the step of applying
pressure to expel excess coating material.
61. The method of claim 60, wherein the pressure is applied with a pressure
roller.
62. A method of forming a capacitor comprising the steps of:
applying a coating to one of an anode foil and an amorphous coating foil,
said coating containing a polymerizable material having a salt and a
crosslinking agent dissolved therein to form a coated foil and an uncoated
foil;
curing the coating; and
assembling the coated foil and the uncoated foil in facing relationship to
form a capacitor assembly.
63. The method of claim 62, wherein the anode foil is coated prior to the
step of assembling the coated foil and the uncoated foil.
64. The method of claim 62, wherein the cathode foil is coated prior to the
step of assembling the coated foil and the uncoated foil.
65. The method of claim 61, further comprising the step of coating the
uncoated foil prior to assembling the foils in facing relationship.
66. A method of forming an electrolytic capacitor comprising the steps of:
providing a three layer assembly having an anode foil, a cathode foil and a
spacer disposed between said anode foil and said cathode foil,
said spacer being ultrathin and having openings occupying at least
substantially twenty percent of its volume;
impregnating the assembly with an electrolyte.
67. The method of claim 66, wherein said electrolyte comprises a
polymerizable material.
68. The method of claim 67, further comprising dissolving a salt in said
polymerizable material.
69. The method of claim 67, further comprising adding a plasticizer to said
polymerizable material.
70. The method of claim 67, further comprising polymerizing the
electrolyte.
71. A capacitor comprising
an anode foil;
a cathode foil,
an ultrathin spacer having openings therein occupying at least
substantially twenty percent of its volume;
an electrolyte impregnated into said openings in said spacer.
72. The capacitor of claim 71, wherein said spacer is comprised of
isotactic polypropylene.
73. The capacitor of claim 72, wherein said spacer has a thickness of less
than 25 .mu..
74. The capacitor of claim 72, wherein said spacer has a microporous
structure.
75. A capacitor comprising:
an anode;
a cathode
a crosslinked polymer electrolyte disposed between said anode and said
cathode;
a sealed housing for containing said anode, said cathode and said polymer
electrolyte, substantially all space in said housing being occupied by
said anode, said cathode and said electrolyte.
76. A capacitor comprising:
an anode;
a cathode;
a crosslinked polymer electrolyte disposed between said anode and said
cathode;
a sealed housing for containing said anode, said cathode and said solid
polymer electrolyte, said housing being devoid of an expansion chamber for
said electrolyte.
77. A capacitor comprising:
an anode;
a cathode;
a crosslinked polymer electrolyte disposed between said anode and said
cathode;
a sealed housing for containing said anode, said cathode and said polymer
electrolyte, said housing being entirely occupied by said anode, said
cathode and said electrolyte except for a longitudinally extending
passageway therein.
78. The capacitor of claim 77, further comprising a hollow forming member
for defining said passageway and wherein said anode and said cathode are
rolled about said forming member.
79. A compact electrolytic capacitor including an electrically conductive
anode, an electrically conductive cathode, and an electrolyte between said
anode and said cathode; characterized in that the electrolyte is in the
form of an ultrathin layer of a solution of (a) at least one salt selected
from the group consisting of organic ammonium salts, zinc salts, cadmium
salts, mercury salts and thallium salts of (b) at least one acid selected
from the group consisting of monobasic, dibasic and tribasic acids other
than haloid acids (c) in an ionically conductive carrier of high solvation
power.
80. A compact electrolytic capacitor including an electrically conductive
anode, an electrically conductive cathode, and an electrolyte between said
anode and said cathode; characterized in that the electrolyte is in the
form of an ultrathin layer of a solution of (a) at least one salt selected
from the group consisting of organic ammonium salts, zinc salts, cadmium
salts, mercury salts and thallium salts of (b) at least one acid selected
from the group consisting of monobasic, dibasic and tribasic acids other
than haloid acids (c) in a carrier of high solvation power; said capacitor
having a breakdown voltage which is increased by at least five percent for
a given construction due to action of said electrolyte.
81. A compact electrolytic capacitor including an electrically conductive
anode, an electrically conductive cathode, and an electrolyte between said
anode and said cathode; characterized in that the electrolyte is in the
form of an ultrathin layer of a solution of (a) at least one salt selected
from the group consisting of organic ammonium salts, zinc salts, cadmium
salts, mercury salts and thallium salts of (b) at least one acid selected
from the group consisting of monobasic, dibasic and tribasic acids other
than haloid acids (c) in a carrier of high solvation power; said capacitor
having a capacitance which is increased by at least five percent for a
given anode construction due to action of said electrolyte.
82. A compact electrolytic capacitor including an electrically conductive
anode, an electrically conductive cathode, and an electrolyte between said
anode and said cathode; characterized in that the electrolyte is in the
form of an ultrathin layer of a solution of (a) at least one salt selected
from the group consisting of organic ammonium salts, zinc salts, cadmium
salts, mercury salts and thallium salts of (b) at least one acid selected
from the group consisting of monobasic, dibasic and tribasic acids other
than haloid acids (c) in a carrier of high solvation power; said capacitor
undergoing, under given elevated temperature and time conditions, only
minimal deforming of its dielectric layer.
83. The capacitor of claim 82, wherein said elevated temperature of
60.degree. C. and said deforming is less than substantially four percent.
84. The capacitor of claim 83, wherein said deforming takes place over a
time of at least 1,000 hours.
Description
TECHNICAL FIELD
This invention relates to electrolytic capacitors. More particularly it
relates to electrolytic capacitors which utilize a solid electrolyte and
to methods of making such capacitors. While the applicability of the
principles of the present invention is fairly wide and general, for the
sake of an orderly presentation, and to facilitate the comprehension of
those principles, the description will focus in the first instance on the
structural characteristics of and manufacturing methods for solid
electrolyte capacitors which are designed to be used in implantable
biomedical electronic devices such as cardiac pacemakers and
defibrillators.
BACKGROUND ART
Cardiac pacemakers and defibrillators to be implanted inside the human body
require associated power supplies which must be provided with a high
capacitance in order to be able to deliver intense bursts of current for
very short time intervals on demand. That electrolytic capacitors are well
suited for performing this function in biomedical electronic devices such
as pacemakers and defibrillators is well known. Given the environment
within which such a device is used, however, it is essential that the
volume of the device be kept to an absolute minimum. Thus, since the
capacitor in such a device ordinarily occupies as much as about 30% of the
total volume of the device, which is a very high proportion relative to
the other electronic components in the device, considerable effort has
been expended on the problem of reducing the size of the capacitor as the
best way for achieving a reduction in the size of the device as a whole.
Nevertheless, attempts to minimize the volume of electrolytic capacitors
have met with only limited degrees of success, for a number of reasons.
Conventionally, a capacitor of this type includes an etched aluminum foil
anode, an aluminum foil or film cathode, and an interposed Kraft paper or
fabric gauze spacer impregnated with a solvent-based liquid electrolyte.
The entire laminate is rolled up into the form of a substantially
cylindrical body and encased, with the aid of suitable insulation, in an
aluminum tube which is enclosed with the other electronics in a
hermetically sealed case of a suitable metal (such as titanium, for
example) inert to body fluids. However, Kraft paper or gauze fabric are
inherently relatively thick. Thus, the presence of those materials will
control the ultimate thickness of the rolled up laminate constituted by
the anode, the cathode and the paper or gauze spacer, i.e., it will limit
the extent to which the size of the capacitor can be reduced.
In any such electrolytic capacitor, of course, there exists the risk that
the liquid electrolyte will leak out. Accordingly, the capacitor must be
hermetically sealed to prevent any leakage of the liquid electrolyte
therefrom, since if the liquid were to come into contact with the other
electronic components encapsulated in the device, it could damage them
sufficiently to cause the device to fail to operate properly. In extreme
cases, the patient's life could then be in jeopardy. Hermetically sealing
the liquid electrolyte into the capacitor thus has become standard
practice, but this also inherently seals in any gases that may become
liberated during the use of the capacitor. To accommodate such gases and
prevent a potentially harmful buildup thereof, it has become necessary to
provide the capacitor with an expansion or compliance chamber into which
the gases can be permitted to escape and accumulate so as to avoid their
having any adverse effect on the device. That, however, has entailed an
increase, rather than a reduction, in the volume of the capacitor and is
clearly an unacceptable expedient for use in a device for which
minimization of volume is a critical consideration.
The presence of the liquid electrolyte in such a capacitor entails yet a
further disadvantage. As is well known, the face of the aluminum anode is
coated with a thin layer of aluminum oxide, which constitutes the
dielectric for the capacitor and is formed through an electrochemical
action resulting from the application of a positive voltage to the anode.
The continued contact of the oxide layer with the solvent-based liquid or
gel electrolyte over a period of time, however, especially while the
capacitor is not in use, tends to cause the oxide layer to become degraded
or "deformed" by being dissolved in the electrolyte, as a consequence of
which the shelf life of the capacitor is relatively limited. Ordinarily,
of course, the application of a voltage across the capacitor during use
would tend to cause the oxide layer to be re-formed, however, the presence
of the liquid electrolyte reduces the lifetime of the formed oxide layer.
Thus, such a capacitor, in addition to a decreased shelf life, tends to
have a shortened useful service life as well.
Among the attempts to achieve a reduction of the volume of such
electrolytic capacitors is one represented by U.S. Pat. No. 3,555,369,
which suggests the replacement of the conventional Kraft paper spacer or
insulator of the capacitor with a thin semipermeable membrane of a
polymeric material. Such a membrane would be thin, i.e., less than 40 .mu.
thick, and preferably its thickness would be between about 1 .mu. and 2
.mu. or even less. Viewed in the abstract, this proposal might well have
enabled a substantial reduction in the volume of the capacitor to be
achieved because, given the normal thickness, on the order of about 100
.mu. or so, of the aluminum foil components of the capacitor, the size of
the rolled up laminate would in essence be determined by the thickness of
the foils, with the contribution of the semipermeable membrane layer to
the overall thickness being, for all practical purposes, negligible.
However, a capacitor according to this proposal requires that the
semipermeable membrane must be impregnated with a solvent-based liquid
electrolyte. Thus, the electrolytic capacitor of this patent must still be
sealed hermetically in order to prevent any leakage of the electrolyte
from the capacitor, and that in turn necessitates the provision of an
expansion or compliance chamber to accommodate any liberated gases. The
provision of such a chamber, of course, negates the volume reduction
achieved by the use of the thin spacer constituted by the semipermeable
membrane. Further, the presence of the liquid electrolyte in the
electrolytic capacitor according to this patent will subject the capacitor
to the previously described deformation of the oxide dielectric layer on
the anode, and at the same time the presence of the liquid electrolyte
will tend to adversely affect the lifetime of the formed oxide layer of
the capacitor.
Starting from another vantage point, it has been proposed in U.S. Pat. No.
3,883,784 to produce capacitors in which the spacer or insulator between
the anode and the cathode does not include a liquid or gel electrolyte but
rather is at least in part a solid "polymeric association product" which,
as disclosed in the patent, is a class of polymeric materials
characterized by a multiplicity of ionic acceptors and a multiplicity of
ionic donors (or interstitial impurities which act as ionic donors). The
polymeric material is preferably an association product of polyethylene
oxide (providing proton acceptor hydrogen bonding sites) and a polymeric
resin such as a phenolic compound (providing proton donor hydrogen bonding
sites), and it is suggested in the patent that this material may behave,
in many aspects, like a solid electrolyte.
U.S. Pat. No. 3,883,784 discloses that the polymeric association product
either may be impregnated into a conventional Kraft paper spacer before
the latter is assembled with the metallic anode and cathode, or may be
formed as a layer or film interposed (without any associated layer of
paper) between the anode and cathode. However, apart from the case of a
capacitor with a Kraft paper spacer (which is inherently subject to the
limitation on capacitor volume reduction previously referred to herein),
the patent further discloses that a film or layer of the polymeric
association product when used as the spacer in a capacitor is on the order
of about 0.0045 inch to about 0.0085 inch thick (approximately 114 .mu. to
216 .mu.). Thus, the polymeric association product spacers which are
described in this patent are far thicker than conventional Kraft paper
spacers, and consequently will not only fail to achieve a volume reduction
for the capacitor but actually will tend to make the same larger than one
utilizing a Kraft paper spacer.
Moreover, notwithstanding the suggestion that some of the various types of
capacitors described in U.S. Pat. No. 3,883,784 may act like electrolytic
capacitors in certain cases, they are clearly not electrolytic capacitors
as that term is understood in the art and do not have the properties of
those types of electrolytic capacitors which are suited for use in
biomedical electronic devices such as pacemakers and defibrillators. This
conclusion is implicit in the fact that the capacitors described in the
patent and utilizing a spacer film made of the stated polymeric
association product material may be bidirectional rather than polar
devices. Thus, such a spacer film will then not be capable of supporting
normal electrolytic action at any overvoltage, and placing a high negative
voltage on the anodized aluminum electrode will reduce the oxide layer,
producing aluminum and, in the presence of the hydrogen ions, hydroxyl
ions, all without the capacitor having any substantial oxide layer
reforming capability. Also, the capacitance values characterizing the
capacitors described in the patent are much smaller than those of normal
electrolytic capacitors of comparable size. Finally, the DC conductivity
of the polymeric association product material used in forming the spacer
films of those capacitors is extremely low for any material ostensibly
functioning as an electrolyte.
DISCLOSURE OF INVENTION
It is an object of the present invention, therefore, to provide novel and
improved electrolytic capacitors which, by virtue of their structural
features, avoid the hereinbefore mentioned as well as other drawbacks and
disadvantages of heretofore known electrolytic and quasi-electrolytic
capacitors designed for the same purposes.
It is a more specified object of the present invention to provide novel and
improved electrolytic capacitors of the aforesaid type the structure of
which is characterized by the presence, between the anode and the cathode
thereof, of a layer of solid electrolyte constituted of a solid solution
of certain metal salts in a polymer matrix, whereby such capacitors are
characterized by being immune to any leakage of electrolyte, by having a
volume appreciably smaller than that of the heretofore smallest available
electrolytic capacitors of comparable constructional and operational
properties, and by having improved electrolytic stability and oxide layer
re-forming properties leading to a longer shelf life as well as a longer
useful service life and a relatively higher working voltage.
It is yet another object of the invention to provide a crosslinked solid
polymer electrolyte.
It is also an object of the present invention to provide methods for making
such solid electrolyte capacitors and the crosslinked solid polymer
electrolyte used therein.
Generally speaking, the objectives of the present invention are attained by
a compact electrolytic capacitor which includes, between the electrically
conductive anode and cathode thereof, an ultrathin layer constituted of a
solid electrolyte comprising a solid solution of (a) at least one salt
selected from the group consisting of alkali metal salts, transition metal
salts, ammonium salts, zinc salts, cadmium salts, mercury salts and
thallium salts of (b) at least one acid selected from the group consisting
of monobasic, dibasic and tribasic acids other than hydrohalic (haloid)
acids (c) in a polymer of high solvation power. As used herein, the term
"ultrathin" designates a spacer the thickness of which is in the range of
about 1 .mu. to about 50 .mu. and preferably is in the range of about 5
.mu. to about 20 .mu..
More particularly, the solid electrolyte according to the present invention
is made from a polymer with polar groups capable of imparting to the
polymer a high solvation power for the dissolving of ions. To this end,
the present invention contemplates production of the electrolyte from a
high solvation power polymer such as a homopolymer, or a block, graft or
other copolymer, or a blend, or a crosslinked polymer of:
siloxane-alkylene oxide copolymers (block, graft and comb); poly(ethylene
oxide), poly(propylene oxide), and other poly(alkylene oxide)s:
poly(poly(ethylene oxide)methacrylate) and other polymers formed from
poly(alkylene oxide) esters of methacrylic acid, acrylic acid and
alkacrylic acids generally; poly(alkylene oxide)-poly-(urethane urea)
copolymers; comb polymers having a polyphosphazene backbone and
poly(alkylene oxide) sidechains; polymers of the poly(alkylene oxide)
esters of itaconic acid; polyepichlorohydrin; poly(ethylene succinate),
poly(Beta-propiolactone), poly(ethylene adipate), and other polyesters;
poly(ethylene imine); poly(N-propylaziridine); or any of the above in
which the poly(alkylene oxide) is replaced by poly(alkylene sulfide).
Plasticization of the polymer to facilitate its formation into a film or
foil is achieved by blending the polymer with up to 60 wt % of a lower
molecular weight compound such as poly(ethylene glycol), poly(propylene
glycol), and other poly(alkylene glycol)s; any of such materials which
have been end-capped with alkoxy, carboxylic acid or other functional
groups; propylene carbonate; or glycerol, ethylene glycol and other
polyols which optionally may be end capped with alkoxy, acid or other
functional groups. Presently preferred is a blend of poly(ethylene oxide)
and a poly(methylmethoxy[poly(ethylene glycol)] siloxane) copolymer.
Dissolved in the polymer and constituting the electrolytic or ion-producing
component of the solid electrolyte, is a salt of an alkali metal such as
lithium, sodium or potassium, or a transition metal such as silver or
mercury, or ammonium, of an acid other than a haloid or hydrohalic acid,
and in particular of a monobasic acid such as cyanic acid, or a dibasic
acid such as carbonic or chromic acid, or a tribasic acid such as boric or
phosphoric acid, or a carboxylic acid such as acetic acid, or a
dicarboxylic acid such as glutaric, oxalic, phthalic or tartaric acid, or
a tricarboxylic acid such as citric acid. Presently preferred salts for
use in the case of a capacitor having an aluminum anode are the
hexafluoroglutarates and the tetrafluoroborates of lithium, sodium and
potassium, the preferred salt is potassium. The molar concentration of the
salt in the polymer electrolyte preferably is between 0.005 and 1 times
that of the oxygen or sulfur atoms therein.
The polymer electrolyte is crosslinked by an agent which may be a di-, tri
or a polyisocyanate and may be selected from the group consisting of
hexamethylene diisocyanate, toluenediisocyante and methyl
diphenylisocyanate. The crosslinking agent may also be
poly(alkylhydrogensiloxane) or a di- or multifunctional reagent which is
an analog of the compound to be crosslinked, and may be selected from the
group consisting of divinyl, diallyl, dialkacrylic and diacrylic analogs
of the compound to be crosslinked The crosslinking agent may also be
selected from the group consisting of di- and multifunctional acids or the
group consisting of di- and multifunctional amines.
The anode preferably is in the form of a foil of any of a class of metals
such as aluminum, tantalum, niobium, tungsten or other anodic metals which
are commonly used in the construction of electrolytic capacitors, aluminum
foil annealed and deeply etched to maximize its surface area being
particularly suitable for the type of capacitor intended for use in
cardiac pacemakers and defibrillators. The layer constituted of the solid
electrolyte in such a capacitor may be constructed by progressively
impregnating and coating the anode foil with increasingly viscous films of
the polymer or a solution thereof in a suitable solvent such as
acetonitrile. This process ensures complete coverage of the finely etched
anode foil (including complete penetration and filling of the etched in
pores or depressions) and provides appropriate stress relief to prevent
powdering at the anode metal/oxide interface or fracture of the foil. The
surface of the solid electrolyte polymer may additionally be crosslinked
by chemical or radiative means. This strengthens the polymer film (or the
polymer foil, if a foil is used), allowing it to be rolled with the anode
and cathode.
The cathode, which in the basic embodiments of the present invention is
made of any suitable metal such as platinum, silver, gold, nickel,
aluminum, or the like, can be constructed in the form of a foil of such a
metal and then laminated with the anode and the solid electrolyte layer.
Alternatively, it can be formed in situ as a film by painting, sputtering,
evaporating, or otherwise depositing the metal directly onto that surface
of the solid electrolyte layer which is directed away from the
polymer/oxide layer interface. It is also contemplated by the present
invention that the cathode can be constituted of a mass of electrically
conductive carbon or other conducting particles in the range of about 0.5
.mu. to 5.0 .mu. in size deposited in layer formation on the surface of
the solid electrolyte layer by means of a suitable polymer ink, or that it
can be constituted of a mass of such particles incorporated in layer
formation and at a loading of at least 50% in the immediate subsurface
region of the solid electrolyte layer. In the first of these variants,
either no such particles are found within the solid electrolyte layer, or
there may also be a quantity of such particles incorporated in the
immediate subsurface region of the solid electrolyte layer but at a
loading of less than 50% so that those particles do not constitute a part
of the cathode but serve to reduce the internal impedance of the
capacitor. In the second variant, of course, a suitable cathode lead will
be provided in electrical contact with the embedded mass of particles.
The dielectric, i.e., the oxide of the anode metal, can be formed before or
after application of the solid electrolyte to the anode. The method of
forming the oxide layer after application of the solid electrolyte entails
exposing the electrolyte polymer, before it is hardened, to a controlled
humid atmosphere so as to cause a limited amount of moisture to be
absorbed by the polymer, and then applying an appropriate electric field
between the anode and the cathode. The polymer will then comprise a small
amount of a hydroxyl group-containing substance; which may be water.
Alternatively alcohol may be used. Preferably, the humidity control is
effected by means of a suitable desiccant, e.g., silica gel or phosphorus
pentoxide, incorporated in the housing of the capacitor. The presence of
the desiccant additionally helps to minimize the subsequent oxide layer
deformation rate while concurrently helping to minimize any decrease in
the oxide layer reforming capability of the capacitor.
As previously intimated herein, it is also an object of the present
invention to provide novel and improved solid electrolyte capacitors which
are adapted for use in applications other than pacemakers and
defibrillators, and also to provide methods of making such capacitors.
Merely by way of example, it is contemplated in accordance with one
modified embodiment of the present invention to provide such a capacitor
which includes an anode that is not made of an etched foil of the anodic
metal but rather is made of a mass of particles of the metal embedded, in
a suitable layer configuration, in a matrix of solid polymer electrolyte.
In this construction, the metal particles may be in the form of flakes,
powder or microspheres and are in electrical contact with one another
throughout the layer-forming mass, with each particle being coated by a
thin dielectric oxide layer. The overall thickness of the polymer matrix
is somewhat greater than that portion thereof which accommodates the
anode-forming mass of particles, thereby to provide an excess quantity of
the solid electrolyte which is free of the metal particles and which is to
constitute the ultrathin layer of electrolyte between the anode and the
cathode. The ultrathin electrolyte layer portion of the polymer matrix may
be formed either jointly with or after completion of the impregnation of
the mass of metal particles in the polymer.
To facilitate the application of potential to the anode, an anode connector
lead in the form of a foil, wire or other suitable electrical conductor is
electrically connected to the mass of anode-forming particles, for
example, by having a portion of the particulate mass protrude from the
electrolyte polymer matrix so as to be able to be connected directly to
the lead, or by having a thin layer of an insulating polymer over the
electrolyte polymer matrix at the location of the particulate anode and
filled with a sufficient quantity of the anodic metal particles to provide
an electronic connection between the anode connector lead and the anode
without providing an ionic connection therebetween. Such a capacitor is
particularly suitable for use in applications where a stacked
configuration, including a plurality of individual capacitance units each
including an anode, a cathode and an interposed solid electrolyte layer,
is desired.
It is further contemplated that in accordance with yet another embodiment
of the present invention the principles thereof may be embodied in a
capacitor, known as a double layer capacitor, which does not include an
anode made of anodic metal and hence does not include an oxide dielectric
layer. Such a capacitor utilizes a porous mass of electrically conductive
carbon particles embedded in a layer formation in a solid polymer
electrolyte matrix to constitute the anode of the capacitor. The particles
may be loose or sintered into the form of a porous body. An anodic
connector foil is electrically connected to the layer of carbon particles,
and an ultrathin layer of solid polymer electrolyte devoid of carbon
particles is interposed between the latter and the cathode. The degree of
impregnation of the mass of carbon particles by the solid polymer
electrolyte is such that each comprises about 50% by volume of the total
anodic body. A capacitor of this type, since it does not use anodic metal
and hence does not include a dielectric layer, is not suited for use in a
high frequency or high voltage environment but is capable of providing a
high capacitance and low voltage capability which makes it suitable for
use in a variety of applications, e.g., a miniaturized computer memory
backup.
The solid electrolyte capacitors according to the present invention provide
a number of advantages.
Among these are:
1. Better linearity of capacitance with applied voltage. Generally,
conventional aluminum electrolytic capacitors have an energy storage value
or capacitance which increases with applied voltage. This is probably due
to penetration of the liquid electrolyte into the aluminum oxide surface
coating on the anode. Sometimes, however, such penetration is undesirable,
as it can result in a change in the dielectric characteristics and hence
in a distortion of the waveform in pulse applications. Because the
capacitor of the present invention does not use a liquid solvent in the
electrolyte, any tendency toward variation of capacitance with applied
voltage is greatly reduced.
2. Higher breakdown voltage. All other things being equal, the solid
electrolyte capacitor of the present invention is characterized by a
higher breakdown voltage than is normally found in conventional
electrolytic capacitors. An improvement in excess of 5% in breakdown
voltage over, for example, Rubycon.RTM. capacitors results by virtue of
the action of the electrolyte of the present invention. For example, if a
capacitor made with a high purity etched aluminum foil and a conventional
liquid electrolyte has a dielectric breakdown voltage of 350 volts DC, an
identically constructed capacitor utilizing a solid electrolyte according
to the present invention would provide an increase in the breakdown
voltage to 390 volts, and even voltages in excess of 400 volts are
achievable at acceptable leakage currents. Furthermore, if a 100 .mu.
thick layer of solid electrolyte is used, the breakdown voltage for the
dielectric can be increased from the nominal rating of 350 volts.
3. Higher capacitance per unit area. In the capacitor of the present
invention, the dielectric oxide layer cannot be hydrated or otherwise
penetrated by the electrolyte; thus, a more compact oxide layer results.
This produces a higher capacitance per unit area of etched anode foil than
is obtained with liquid electrolytes at a given voltage. An improvement in
excess of five percent over, for example, Rubycon.RTM. capacitors results
by virtue of the electrolyte of the present invention. For example, a
segment of foil having a discharge capacitance of 0.74 .mu.F per cm.sup.2
in a liquid electrolyte is found to have a discharge capacitance of 0.87
.mu.F per cm.sup.2 in the solid polymer electrolyte of the present
invention. Solid polymer electrolyte capacitors according to the present
invention are further found on discharge to deliver significantly more
charge than would normally be expected on the basis of the anode foil
surface area in a liquid electrolyte capacitor.
4. Lower electrical leakage and improved shelf factor. In a conventional
capacitor provided with a liquid or gel electrolyte, electrical leakage
increases with age because of parasitic electrochemical reactions which
break down the insulating oxide layer coating the anode. The solid
electrolyte capacitors of the present invention have a much lower
electrical leakage at any given voltage and are much less susceptible to
dielectric layer deformation because of the absence of a liquid
electrolyte. Consequently, the increase of electrical leakage with age of
such capacitors is substantially reduced and their shelf life is
materially enhanced. For example, for capacitors using an electrolyte
composition according to the invention including sodium tetrafluoroborate,
in tests at 60.degree. C. for 1,000 hours, deforming of the oxide layer
may be limited to 4% as opposed to up to 40% for prior art capacitors of
the Rubycon.RTM. type.
5. Reduced capacitor size Leakage of the liquid or gel electrolyte from a
conventional electrolytic capacitor is a significant problem in high
reliability applications. For such an application it is necessary,
therefore, to hermetically seal the capacitor housing to prevent the
electrolyte from leaking out, and the housing also requires an additional
free volume serving as a compliance or expansion chamber for evolved
hydrogen. In a capacitor according to the present invention, the
electrolyte, being solid, does not leak or diffuse out of the capacitor,
and the capacitor housing thus does not need to be hermetically sealed.
The size of the capacitor is, consequently, greatly reduced vis-a-vis that
of a conventional liquid electrolyte capacitor, which is a substantial
advantage where the capacitor is intended for use in a cramped
environment, e.g., in an implantable biomedical electronic device such as
a pacemaker or defibrillator, or in a miniaturized computer memory device,
or the like.
6. High discharge current density and low dissipation factor. The present
invention enables very high current discharges and low dissipation factors
to be achieved in a completely solid electrolyte because of the presence
therein of a highly polar polymer such as the siloxane-alkylene oxide
copolymers which allow the solvation of alkali metal salts (such as, for
example, potassium hexafluoroglutarate, sodium tetrafluoroborate, lithium
thiocyanate, and the like).
7. Improved utilization of anode foil strength. The solution of the solid
electrolyte of this invention in acetonitrile has a low viscosity before
curing and can penetrate pores less than 0.025 .mu. wide. Thus, the highly
etched foil used in aluminum electrolytic capacitors, which is normally
quite fragile and brittle, can be totally penetrated with a coating of
polymer on all pore surfaces. This takes full advantage of the foil's high
surface area, strengthens it, and reduces the tendency for the foil and
its oxide coating to powder. Local stress relief can be provided by
applying the polymer electrolyte to the foil in a succession of passes,
with the polymer in the successive layers being of progressively increased
molecular weight.
BRIEF DESCRIPTION OF DRAWINGS
The foregoing and other objects, characteristics and advantages of the
present invention will be more clearly understood from the following
detailed description thereof when read in conjunction with the
accompanying drawings, in which:
FIG. 1 is a diagrammatic fragmentary cross-sectional view of the basic
laminate structure of the components of a solid electrolyte capacitor
according to one embodiment of the present invention;
FIG. 1A is a diagrammatic fragmentary cross-sectional view, similar to FIG.
1, of the laminate structure of the components of a solid electrolyte
capacitor according to a slightly modified embodiment of the present
invention;
FIG. 1B is a view similar to FIG. 1A but is drawn to a greatly enlarged
scale to provide a schematic illustration of the etching of the anode foil
in a capacitor structure according to FIGS. 1 and 1A;
FIG. 2 is a schematic illustration of a solid electrolyte capacitor the
capacitance element of which is constituted by the laminate of FIG. 1 or
FIG. 2 rolled up into the form of a cylindrical body;
FIG. 3 is a diagrammatic fragmentary section through a stack-type solid
electrolyte capacitor according to a modified embodiment of the present
invention, the view being taken along the line III--III in FIG. 4;
FIG. 4 is a plan view of the structure shown in FIG. 3, the view being
taken along the line IV--IV in FIG. 3;
FIG. 5 is a diagrammatic fragmentary section through a solid electrolyte
capacitor utilizing an anode constituted of a layer of solid electrolyte
polymer-impregnated metallic particles according to a further modified
embodiment of the present invention;
FIG. 6 is an enlarged detail view of the structure enclosed within the
circle VI in FIG. 5;
FIG. 7 is a diagrammatic fragmentary section through a solid electrolyte
double layer capacitor utilizing an anode constituted of a porous layer of
electrolyte polymer-impregnated sintered carbon particles according to yet
another modified embodiment of the present invention;
FIG. 8 is an enlarged detail view of the structure enclosed within the
circle VIII in FIG. 7;
FIG. 9 is a diagrammatic fragmentary section through a solid electrolyte
double layer capacitor similar to that shown in FIG. 7 but illustrating a
somewhat different manner of forming the anode;
FIG. 10 is a diagrammatic fragmentary section through a solid electrolyte
double layer capacitor similar to that shown in FIG. 9 but illustrating
yet another manner of forming the anode;
FIGS. 11, 11A and 11B are diagrammatic fragmentary cross-sectional views of
the basic solid electrolyte capacitor laminate structure utilizing a novel
cathode construction according to still another embodiment of the present
invention;
FIG. 12 is a partially cut away, perspective view of an assembly used to
produce yet another embodiment of the invention; and
FIG. 13 is a simplified cross-sectional view of yet another embodiment of
the invention.
MODES FOR CARRYING OUT THE INVENTION
Referring to FIG. 1, there is shown in greatly exaggerated form the basic
laminate or sandwich layer structure 10 which constitutes the capacitance
element of a solid electrolyte capacitor embodying the fundamental
principles of the present invention The laminate includes a highly etched
foil 11 (the etching is not shown in this view) of a suitable anodic
metal, preferably aluminum, to constitute the anode of the capacitor, the
foil 11 having on both its opposite faces respective thin layers 12 and 13
of aluminum oxide (or the oxide of whatever other metal the foil 11
happens to be made of) to constitute the dielectric of the capacitor.
Overlying the dielectric layers 12 and 13 and filling the etched in pores
of the anodic foil 11 are respective ultrathin layers 14 and 15 of the
solid electrolyte according to the present invention. Overlying the solid
electrolyte layer 14 is a further foil or film 16, of a metal such as
aluminum, silver, gold, platinum, nickel, or the like, to constitute the
cathode of the capacitor.
In the preferred embodiment of the present invention, the solid electrolyte
is in the form of an ultrathin layer not more than about 50 .mu. thick,
and preferable not more than about 20 .mu. thick. The electrolyte is
composed of a solid solution of a metal salt in a polymer of high
solvation power, preferably a solid solution of an alkali metal salt of a
dicarboxylic acid, e.g., potassium hexafluoroglutarate, in a blend of a
siloxane-alkylene glycol copolymer with poly(ethylene oxide). The solid
electrolyte layers 14 and 15 could in theory be separately prepared as
foils and then bodily laminated to the dielectric-coated anodic foil 11.
In practice, however, it is found that the handling of an ultrathin layer
of polymer electrolyte in the form of a foil poses substantial
difficulties. It is preferred, therefore, to form the layers 14 and 15 as
films by depositing the polymer electrolyte a solution thereof in
acetonitrile directly onto the surfaces of the anode foil and then curing
the films. Within this concept the fluid polymer may be applied to each
anode surface in several passes, with the polymer in each pass after the
first having a somewhat higher molecular weight than the polymer in the
last preceding pass. Thus, the film of lowest molecular weight would be
applied first and thereafter the additional films of progressively higher
molecular weight. This manner of progressively building up the final solid
electrolyte layer ensures the complete coverage of the entire surface of
even the most finely etched anode foil, since the solution of the polymer
electrolyte is able to penetrate even into pores less than 0.025 .mu.
wide, and provides appropriate stress relief to prevent powdering at the
anode metal/oxide interface or fracturing of the anodic foil.
In essence, the application of the polymer electrolyte film may be effected
in either of two ways. One of these entails utilizing for the various
passes respective polymers which have different molecular weights prior to
their polymerization; for example, a coating of an electrolyte-containing
solution of polymerizable material of low molecular weight is applied to
the bare oxidized surface of the anode foil in an amount sufficient to at
least fill all the pores thereof, and the material is then polymerized,
after which a second coating of an electrolyte-containing solution of
polymerizable material of high molecular weight is applied to the hardened
first coat, in an amount sufficient to bring the overall electrolyte layer
thickness relative to the foil surface to the desired value, and followed
by the polymerization of the second material. The other way entails
utilizing for the various passes an electrolyte-containing solution of
polymerizable material the respective quantities of which have been
prepolymerized to different degrees and thus have correspondingly
different molecular weights, with the less polymerized material being
applied first.
Other methods of obtaining the same result may, of course, also be used.
Merely by way of example, the anode foil may be coated with an
electrolyte-containing solution of polymerizable material applied to the
desired thickness in a single pass, which material is then provided with a
degree of surface polymerization (e.g., by means of surface irradiation
with ultraviolet light) which is greater than its bulk polymerization.
It will be understood that the choice of any particular one of these
methods will in general depend on the ultimate design characteristics of
the capacitor desired. Thus, the use of any particular degree of variation
of the molecular weight of the polymer throughout the thickness of the
solid electrolyte layer will enable the manufacturer to control the
internal resistance of the capacitor relative to the strength of the
electrolyte layer.
It will further be understood that a degree of control over the ultimate
characteristics of the capacitor may also be achieved by an appropriate
selection of the polymer components of the solid electrolyte. Thus, using
only poly(ethylene oxide) by itself as the polymer component of the solid
electrolyte would provide a layer having higher mechanical stability at
high temperatures (90.degree. C.), but such a layer would also have a high
internal electrical resistance. On the other hand, utilizing only a
siloxane copolymer as the polymer component of the solid electrolyte would
provide a layer having a low internal electrical resistance, but such a
layer would also have a relatively low strength at high temperatures
(above 60.degree. C.). A blend of these polymers would, of course, yield a
layer in which these properties are modified correspondingly, and the
particular composition chosen in any given case will have to be determined
in light of the intended application for and use environment of the
capacitor. Merely by way of example, however, it is believed that a
60%/40% blend of poly(ethylene oxide) and siloxane-alkylene glycol
copolymer will provide optimal characteristics for most applications.
The cathode layer 16 may also be provided in the form of a separate foil
suitable for lamination to the solid electrolyte layer 14. Alternatively,
however, especially if the cathode thickness is to be minimized, the
cathode layer may be formed in situ as a film by painting, sputtering,
evaporating or otherwise depositing the metal onto the surface of the
solid electrolyte layer. Merely by way of example, the cathode may be
formed by applying to the hardened solid electrolyte layer a film of a
polymer ink containing fine particles of a conductive metal such as silver
or nickel in suspension, or the film of silver or other cathodic metal may
be formed by sputtering, chemical deposition or vapor deposition of the
metal onto the solid electrolyte surface. A further variant of the
formation of the cathode will be more fully described hereinafter with
reference to FIGS. 11, 11A and 11B.
The laminate 10' shown in FIG. 1A is essentially the same as that shown in
FIG. 1 except for the additional provision of a second cathode layer 16a
substantially identical to the cathode layer 16. The layer 16a is
advantageously adapted to be connected electrically in parallel to layer
16 to decrease the internal resistance and also provide a small increase
in capacitance. Further description of the structure shown in FIG. 1A, in
which elements corresponding to those of FIG. 1 are identified by the same
reference numerals, is not necessary.
FIG. 1B schematically illustrates the surface detail of the anode foil 11.
Such foils are commercially available products and as such do not
constitute a part of the invention. Accordingly, neither their actual
structures (both micro and macro) nor the methods of production thereof
are described herein. It is deemed sufficient to point out that the foil
is annealed, and the opposite surfaces 11a and 11b of the foil 11 are
deeply and finely etched to provide a multiplicity of microscopic pores or
depressions 11c and 11d in the respective surfaces. As indicated in FIG.
1B, some of these pores are deeper and/or wider than other pores and, as
represented at 11e, it is actually preferred that at least some of the
pores extend through the entire thickness of the foil. FIG. 1B also shows
that the dielectric oxide layers 12 and 13 coat the respective surfaces of
the foil 11 each throughout its entire expanse, including over the entire
depths of the various pores. It should be understood, of course, that the
illustration of the laminate in FIG. 1B is not intended to indicate
precisely the forms and configurations of the etching in the anode foil
11.
As previously mentioned, the provision of the pores or depressions 11c, 11d
and 11e not only maximizes the available surface area of the foil but also
enables the solid polymer electrolyte, when the same is being applied in
solution to the opposite surfaces of the foil to form the electrolyte
layers 14 and 15, to penetrate into the pores. As a result, a degree of
mechanical interlocking of the electrolyte layers with the anode foil 11
is provided as well as a degree of stress relief through which a
deterioration of the oxide coatings 12 and 13 at the interfaces thereof
with the metal surfaces 11a and 11b of the foil and a possible fracturing
of the latter are inhibited.
Referring now to FIG. 2, the electrolytic capacitor 17 there shown, the
design of which is such as to render the capacitor suited for use in a
pacemaker or defibrillator, utilizes as its capacitive element a laminate
such as that designated 10 in FIG. 1 or preferably that designated 10' in
FIG. 1A. To this end, the laminate, with suitable leads 18 and 19
appropriately secured to the anode and cathode foils or films 11 and
16/16a, respectively, is wound onto a thin, e.g., 3 mm diameter, cardboard
core tube 20 into the form of a compact, substantially cylindrical body
21. The entire assembly is inserted into a cylindrical housing 22 of
aluminum, titanium, or like inert stable metal, with the exterior surface
of the cylindrical body 21 being insulated from the housing by an
interposed insulating spacer 23. Housing 22, which is closed at its bottom
end 22a, is sealed at its top end 22b in any appropriate manner, such as,
for example, by means of an end plate 24 which is secured in place by
crimping or rolling of the metal housing or by being brazed thereto.
Suitable openings (not shown) are provided in the plate 24 through which
the leads 18 and 19 pass with an insulative hermetic seal. For purposes of
moisture control, i.e., to minimize the likelihood of electrolytic attack
on the oxide dielectric layers in the event of the presence of some
residual quantities of moisture in the housing and particularly in the
electrolyte layers, it is preferred to enclose a quantity of a suitable
desiccant (not shown), such as silica gel, phosphorous pentoxide, or the
like, in the core tube 20.
Referring to FIGS. 3 and 4, the capacitor 25 there illustrated includes a
plurality of capacitive units 26 in a stacked arrangement (two are shown
but there could be more). Each unit 26 includes an anode foil 27 deeply
etched to provide maximum surface areas and coated on both its faces with
respective ultrathin solid electrolyte layers 28 and 29, and a respective
cathode foil or film 30 overlying the face of the associated electrolyte
layer 28 directed away from the associated anode 27. As in the previously
described embodiments of the present invention, the cathodes may be in the
form of foils or films. In effect, therefore, the anodes and cathodes are
interleaved, with the edges of the anodes not being adjacent the edges of
the cathodes to reduce the field strength at the edges of the anodes and
minimize the chance of an electrical breakdown. Suitable means are
provided for connecting the anodes 27 and cathodes 30 to respective
sources of positive and negative potential; in the illustrated embodiment,
these means are in the form of pairs of metallic ribbons 31-32 and 33-34
which are secured to their respective electrode elements, for example, by
being spot welded or riveted thereto as schematically indicated at 35 and
36. Each of the two stacks of ribbons, which at the same time serves to
maintain the desired spacings between the corresponding capacitor
electrodes, is provided with a suitable lead 37 or 38 which extends out of
the housing 39 of the capacitor for connection to an associated power
source (not shown). To further minimize the creation of excessive electric
field concentrations, the solid electrolyte layers 28 and 29 of each unit
are extended around the edges of the associated intermediate anode foil 27
and, as shown at 40, the amount of the polymer electrolyte juxtaposed to
the edge of each anode foil is somewhat increased in thickness to
approximately twice the thickness of the layers 28 and 29 by, for example,
dipping of the coated anode into uncured electrolyte.
In accordance with the embodiment of the invention illustrated in FIGS. 5
and 6, the capacitor laminate structure 40 there shown includes an anode
41 which is constituted, in the preferred form of this embodiment, of a
mass of deeply etched aluminum flakes 42 disposed in a layer formation and
embedded in a matrix of polymeric material 43. The layer of flakes is
covered over its entire expanse with an aluminum oxide dielectric layer,
not shown in FIG. 5 but designated 44 in FIG. 6 where it is also indicated
at 42a that at areas of contact between adjacent ones of the flakes the
latter are not oxide coated to establish a continuous electrical contact
between the flakes throughout the mass. The loading of the flakes 42 in
the polymer 43 at a density of approximately 50% by volume is sufficient
to ensure that flake to flake contact exists and that at least a portion
of the mass of flakes is exposed at one surface of the polymer matrix 43,
as indicated at 43a in FIG. 5, to enable an anode connector lead 45 to be
electrically connected to the anode. When a metal oxide forming lead 45 is
used, no gap 43a is required. Superimposed over the anode-constituting
mass of flakes 42 is an ultrathin layer 46 of solid electrolyte according
to the present invention, to the free surface of which is secured the
usual cathode foil or film 47 as previously described.
Polymer matrix 43 may be constituted of the same polymer electrolyte as the
layer 46. Both the polymer matrix for the mass of particles and the solid
electrolyte layer may be formed in one operation, with the quantity of
polymer electrolyte used being in excess of that needed to impregnate the
mass of flakes by an amount sufficient to form the desired thickness of
the electrolyte layer. If the polymer matrix 43 and the layer 46 are
formed separately, the latter may be built up in several passes of
electrolyte-containing solution of polymerizable material having different
molecular weights as previously described. Although the anode layer 41 has
been described as constituted of a mass of aluminum flakes, it may be
composed of a mass of other types of particles, e.g., powder granules,
microspheres, and the like, and these may be made of anodic metals other
than aluminum.
In accordance with yet another embodiment of the present invention, there
is shown in FIGS. 7 and 8 a solid electrolyte capacitor laminate structure
48 having an anode 49, a cathode film or foil 50 and an intermediate solid
electrolyte layer 51. The anode of the capacitor in this embodiment of the
invention is constituted of a mass of mutually contacting electrically
conductive carbon particles embedded in a 2-layer polymer matrix, the
layer 52 containing the anodic carbon particles 53 being composed of the
solid electrolyte according to the present invention and the layer 54
containing the carbon particles 55 being composed of a nonelectrolytic
polymer insulating compound. Positive potential is applied to the anode
particles 53 via a connector lead 56, e.g., a foil or wire, which is
electrically connected to at least some of the carbon particles 55 exposed
at the free surface of the layer 54. The nonelectrolytic polymer layer 54
ensures that the anode connector 56 is not in direct electrical contact
with the solid electrolyte. Preferably the carbon particles in the anode
layer 49 occupy about 50% by volume of the total layer No carbon particles
are present in the ultrathin layer 51 of solid electrolyte between the
cathode and the anode.
Anodic layer 52 may be formed independently of the solid electrolyte layer
51, with the latter being formed in one or more passes as described above
Alternatively, it is possible to form both the particle-filled layer 52
and the particle-free solid electrolyte layer 51 in one operation, with
the quantity of the polymer electrolyte material used being in excess of
the amount required to impregnate the mass of carbon particles 53 by the
amount required to form the layer 51.
Referring to FIG. 9, the capacitor laminate structure 57 there shown
includes an anode layer 58, an ultrathin solid electrolyte layer 59
according to the present invention, and a cathode foil or film 60. The
anode of the capacitor in this embodiment of the invention is constituted
of a 50% by volume porous body 61 of sintered carbon particles. A certain
portion of the sintered carbon body is impregnated with the solid
electrolyte according to the present invention, the impregnation being
controlled, however, to ensure that a thin region 61a of the sintered
carbon body, to the free face of which the anode connector lead, i.e., the
foil or wire 62, is connected, remains free of the solid electrolyte. The
impregnation of the sintered carbon body 61 is accompanied by application
of the ultrathin solid electrolyte layer 59 to the opposite face of the
anode 58, with the layer 59 being formed either by an excess of the
material impregnated into the carbon body or by a quantity of the polymer
electrolyte applied separately in one or more passes in the manner
previously described herein.
In accordance with yet another embodiment of the present invention, the
capacitor laminate structure 63 shown in FIG. 10 differs from that of FIG.
9 in that it includes an anode layer 64 a part of which is composed of a
porous body 65 of sintered carbon particles impregnated with solid
electrolyte according to the present invention and another part of which
is composed of a body 66 of nonporous carbon interposed between the porous
body and the anode connector lead 67. The other components of the
laminate, i.e., the solid electrolyte layer 68 and the cathode foil or
film 69, as well as the manner of impregnation of the porous carbon body
65 and the manner of formation of the layer 68, are the same as before.
As noted above, the double layer capacitors of FIGS. 7-10 are generally of
very high capacitance per unit volume, but have low voltage ratings, and
are thus particularly suitable for use as computer memory backup.
As previously mentioned, the present invention also contemplates the
formation of the cathode of the capacitor in a novel way by utilizing a
mass of electrically conductive carbon particles. One variant of this
aspect of the invention is illustrated schematically in FIG. 11, which
shows a laminate 70 basically similar to the laminate 10 shown in FIG. 1
and including an anode 11, a pair of oxide dielectric layers 12 and 13
thereon, a pair of solid electrolyte layers 14 and 15 over the oxide
layers, and a cathode. The latter, which is designated 71 in FIG. 11, is,
however, constituted of a layer of electrically conductive carbon
particles 72 in electrical contact with each other. The cathode 71
preferably is formed by depositing (e.g., by painting) onto that surface
of the solid electrolyte layer 14 which is directed away from the anode a
coat of a suitable liquid carrier having the particles 72 dispersed
therein and by thereafter drying, evaporating or otherwise solidifying the
carrier.
Another variant of this aspect of the invention is schematically
illustrated in FIG. 11A. The laminate 70' there shown is essentially
identical to that shown in FIG. 11 except that it has a mass of carbon
particles 73 in a layer formation dispersed in the immediate subsurface
region of the solid electrolyte layer 14 at a loading of somewhat less
than 50%. Here the particles 73 do not constitute a part of the cathode,
however, and except in random occurrences are not in electrical contact
with each other. Rather, they are incorporated in the electrolyte layer
only to reduce the internal impedance thereof. It will be understood that
the layer formation of the particles 73 can be formed in various ways; for
example, this can be done by dispersing the particles in a quantity of the
uncured electrolyte polymer solution and then applying the latter in a
second (or subsequent) pass to the particle-free cured portion of the
layer 14 formed in the first (or prior) pass, following which the
particle-containing polymer is cured, or by printing an ink or liquid
carrier containing the particles onto the still uncured layer of polymer
electrolyte so that the particles can migrate to the desired extent into
the subsurface region of the layer before the same is cured. In either
case, of course, the cathode-constituting layer 71 of particles 72 is
applied to the solid electrolyte after the above-described incorporation
therein of the particles 73 has been completed.
Yet another variant of this aspect of the invention is schematically
illustrated in FIG. 11B. The laminate 70" there shown differs from those
of FIGS. 11 and 11A only in that it has no cathode-forming layer of carbon
particles external to the solid electrolyte layer 14. Rather, the cathode
74 is constituted of a mass of carbon particles 75 incorporated in a layer
formation in the immediate subsurface region of the solid electrolyte
layer 14 at a loading of at least, and preferably somewhat greater than,
50% so that the particles are in electrical contact with each other
throughout the layer formation and simulate a solid conductor The
particles 75 in the cathode 74 must, of course, be capable of being
connected to a suitable cathode lead 76, for example, by virtue of
projecting slightly from the surface of the solid electrolyte layer 14 or
otherwise. The manner of forming the cathode 74 is essentially the same as
the manner in which the impedance-reducing layer formation of the
particles 73 is formed and thus need not be described again at this point.
The manufacture of solid electrolyte capacitors embodying the principles of
the present invention is further explained by the following examples.
EXAMPLE 1
An anode made from a 1 cm .times.1 cm piece of aluminum foil, 100 .mu.
thick and etched with holes 1-2 .mu. in diameter, has an anode connection
spot welded at one edge. The surface of the foil is anodized, and the foil
is then impregnated with a polymer electrolyte comprising potassium
hexafluoroglutarate dissolved in a blend of 50% poly(ethylene oxide)
3.times.10.sup.5 gmol.sup.-1, 40% poly(methylmethoxy[poly(ethylene glycol)
350 daltons siloxane]) and 10% styrene, with the salt being present at a
concentration of 0.04 moles of salt per mole of ethylene oxide repeat
unit. After the polymer has been cured and the surface crosslinked, a
coating of silver paint is applied to the resultant solid electrolyte film
to form the cathode. Connection to the cathode is by a sliver of foil 0.5
cm .times.1 cm, rolled and welded to the cathode lead. The entire assembly
is then exposed to a humid environment controlled by silica gel, and the
aluminum oxide coating to constitute the dielectric is formed by applying
400 volts between the anode and cathode for 24 hours. The final assembly
is oven dried at 60.degree. C. for 24 hours under vacuum and then
encapsulated in a coat of epoxy resin. The resulting capacitor
construction is suited for use as a high voltage, printed circuit
board-mounted capacitor and has a capacitance of about 0.87 .mu.F, a
voltage rating of about 380 volts, and an impulse current rating of 0.5
amp.
EXAMPLE 2
A capacitor is constructed as in Example 1 but the oven drying step is
omitted, leaving a small quantity of moisture absorbed in the polymer
electrolyte. Such a capacitor will have an enhanced oxide layer re-forming
capability.
EXAMPLE 3
An anode is constructed from one piece of highly etched aluminum foil (10
cm .times.44 cm .times.75 .mu.) . The etched pores in the foil are between
1 and 2 .mu. in diameter and penetrate the thickness of the foil. The
anode lead is connected to the foil by a series of rivets. The anode foil
is impregnated with the same electrolyte salt/polymer blend as in Example
1. After the polymer has been cured and the surface of the resultant solid
electrolyte film crosslinked a coating of silver paint is applied to the
film to form the cathode. Connection to the cathode is by a piece of
unetched foil 0.5 cm .times.10 cm, rolled and welded to the cathode lead.
The assembly is then exposed to a silica gel controlled humid environment
and wound onto a 3 mm diameter cardboard tube. The assembly is then
encapsulated in an aluminum tube with a rolled end seal, typical of the
style found in photo flash capacitors. The dielectric is formed by
applying 400 volts to the anode and cathode for 24 hours while the
assembly is exposed to a controlled humidity at 25.degree. C. Finally the
end seal is closed. The resultant device has a discharge capacitance of
about 400 .mu.F, a rated voltage of about 390 volts, and an impulse
current rating of 200 amps.
EXAMPLE 4
Ten sheets of aluminum foil (10 cm .times.10 cm .times.50 .mu.), with
aluminum ribbon spot welded at one edge, are prepared as anodes as in the
preceding example. These sheets are saturated with the same polymer
electrolyte as before but containing no styrene, with a double coat being
provided at the edge of each sheet. After curing, but without
cross-linking of the surface of the solid electrolyte films, the sheets
are stacked and interspersed with cathodic conducting foils (10 cm
.times.9 cm .times.10 .mu.), so that the edges of the anode and cathode
foils are not adjacent. The dielectric oxide is formed as detailed in the
previous examples, and the whole assembly is encapsulated in a box
container. This type of assembly optimizes current capability by use of
cathode foil rather than conductive paint (the former having a higher
conductivity than the latter) and by omitting the crosslinking step which
is not necessary for this construction because a multi-layer stacked film
capacitor, being a flat layered structure, has adequate mechanical
strength.
EXAMPLE 5
Flakes of etched formed aluminum anode material are mixed with the
previously described polymer blend in a proportion of greater than 45%
v/v. The mixture is spread as a thin film (10 cm .times.10 cm .times.100
.mu.), a 10 cm aluminum wire lead is placed along the diagonal of the
square, and the whole assembly is cured. The metal flake-impregnated
polymer is then coated with a layer of electrolyte polymer about 10 .mu.
thick and the polymer cured. Sheets of this material are then interspersed
with sheets of cathodic foil to produce a simply assembled capacitor. The
whole assembly is subjected to the dielectric forming procedure described
for the previous examples. The finished assembly forms a capacitor of
about 200 .mu.F and rated at about 350 volts. This form of construction is
suitable where ease of assembly is the prime objective.
EXAMPLE 6
A high surface area carbon anode 10 cm .times.5 cm .times.100 .mu. in size
is coated with a layer of the solid electrolyte of Example 1, the layer
being 10-50 .mu. thick. A gold foil cathode 10 cm .times.5 cm .times.10
.mu. in size is then applied to the electrolyte layer. The resultant
device constitutes a double layer capacitor having a capacitance of 50 mF
and rated for .+-.5 volts.
CROSSLINKING
The optimum polymer mixture described herein is a mixture of
poly(ethyleneoxide) and a siloxane polyethyleneoxide copolymer. Such
mixtures relied on the crystallinity of the polyethyleneoxide component to
provide the solid-like mechanical properties to the material. However such
an approach to preparing a solid polymer electrolyte has two
disadvantages: first, the material becomes a highly viscous fluid above
its melting point; and second the presence of crystallinity in the solid
polymer electrolyte lowers the conductivity of the material. The above
described methods of solving the former problem included incorporating a
crosslinkable substance such as styrene in the electrolyte and
crosslinking the material after application.
In accordance with the invention there are alternative methods of
crosslinking the polymer electrolyte material so that an elastomeric
substance is obtained which contains no crystallinity and which therefore
has improved conductivity and no temperature limit except that at which
the material begins to decompose.
A number of methods exist for crosslinking solid polymer electrolytes based
on the alkyleneoxide --(CH.sub.2 CHRO)-- repeat unit. These include using
mono-, di-, tri- and/or poly functional hydroxy compounds based on the
alkyleneoxide repeat unit and crosslinking these using a diisocyanate to
produce a polyurethane elastomer. Examples of such hydroxy terminated
alkyleneoxide based compounds include:
##STR1##
where R and R' may be H or an alkyl group. In these compounds the overall
molecular weight can be as high as is consistent with the substance not
crystallizing in the cured elastomer. The isocyanate component of the
polyurethane may be any of the standard di- or polyfunctionl isocynates
used in polyurethane elastomer manufacture including hexamethylene
diisocyanate, toluenediisocyanate and methyl diphenylisocyanate.
Alternatively it is possible to use vinyl, allyl, acrylic, alkacrylic or
other unsaturated functional group terminated compounds based on the
alkyleneoxide repeat unit and crosslinking these by including a small
quantity of some multifunctional unsaturated compound and free radical,
ionic or radiation induced initiation. The multifunctional reagent in
these cases can be the divinyl, diallyl, diacrylic, dialkacrylic, etc.
analogue of the main alkyleneoxide based substance to be crosslinked.
Examples of such unsaturated functional group terminated alkylene oxide
based compounds include:
CXY.dbd.CRCOO--(--CH.sub.2 CHR'O--)--R"
CXY.dbd.CRCH.sub.2 O(--CH.sub.2 CHR'O--)--R"
where X,Y,R and R' are H or alkyl groups and R" is H, an alkyl group, or in
the case of the difunctional compounds, some unsaturated functional group.
Other methods of crosslinking mono-, di- and tri-hydroxy alkyleneoxide
based compounds include the formation of polyesters via reaction with di-
or multifunctional acids, the formation of polyamides via the reaction
with di- or multifunctional amines and the reaction of vinyl, allyl,
acrylic or alkacrylic terminated compounds with
poly(alkylhydrogensiloxane) to produce a siloxane poly(alkyleneoxide)
copolymer.
In all of these methods of crosslinking the solid polymer electrolyte
attention must be paid to the effect of the crosslinking materials and
reaction on the conductivity of the resultant cured solid polymer
electrolyte. In general it is highly desirable to have the content of
oxyalkylene groups as high as possible in the cured solid polymer
electrolyte and the individual oxyalkylene polymer chain lengths should be
as long as possible consistent with there being no tendency for the chains
to crystallize in the cured solid polymer electrolyte. For these reasons
the crosslinked materials described below are different from the
elastomeric materials which are produced using similar curing reactions in
non-capacitor applications. It is notable that such materials would rarely
have sufficient ion solvation power to produce a solid polymer electrolyte
of usefully high conductivity.
In ethylene oxide based polymers, crystallization of the polymer chains can
be suppressed by the inclusion on the polymer backbone of some large
pendant group at frequent intervals. The pendant group operates to disrupt
the crystal packing. There are many pendant groups which will suffice.
Random copolymers of ethylene oxide and propylene oxide represent good
examples of this effect wherein between 6% and 100% of the repeat units in
the polymer chain are propylene oxide groups. The inclusion of the
propylene oxide repeat units is sufficient to cause the depression of the
melting point to a degree that crystallization takes place only with great
difficulty. Such random ethylene oxide - propylene oxide copolymers are
available at various molecular weights from Imperial Chemical Industries
Pty. Ltd. under the trade name Daltolac.
Further improvement in the conductivity of the elastomeric materials which
are the subject of this invention can be achieved by the inclusion in the
material of a liquid plasticizer. The plasticizer must be compatible with
the polymer matrix and must also be a good solvent for the salt being
employed .A range of plasticizers commonly used in the plastics and rubber
industries are suitable, as will be understood by those skilled in the
art. Of particular utility are the low molecular weight liquid alkylene
oxide oligomers and polymers. However, it is important to ensure that the
end groups of these molecules are not hydroxy or amino groups in order
that they are not reacted in the polymer network. For this reason, it is
optimal to block the end groups via substitution of alkyl ethers, alkyl
urethanes and/or esters. In certain applications sufficient plasticizer
may be added to cause the material to become gel-like.
Of the crosslinking methods outlined above, the first two, namely the
production of a urethane crosslink and the production of saturated
hydrocarbon crosslinks, are optimum for use in solid polymer electrolyte
based electrolytic capacitors since neither produces volatile byproducts
and both reactions can be caused to take place at moderate temperatures on
the addition of a catalyst. On the other hand the formation of ester and
amide linkages invariably involves the production of a volatile byproduct
which must be removed if the reaction is to go to 100% completion. In the
following description attention is focused primarily on the urethane based
crosslinking method. However, it should be understood that the other
methods of producing the crosslinked solid polymer electrolyte material
are equally useful in this application when optimal compositions and
processes are established.
A critical part of the formulation of the solid polymer electrolytes is the
nature and concentration of the dissolved salt. Suitable salts, identified
above, include the alkali metal, alkali earth, transition metal, ammonium,
organic ammonium, zinc cadmium, mercury and thallium salts of acids
selected from the group consisting of monobasic, dibasic and tribasic
acids other than the haloid acids. The choice of salt depends on the
properties desired of the electrolyte. For example, where high aluminium
electrolytic capacitor shelf life is required, a suitable salt has been
found to be NaBF.sub.4. On the other hand, where electrical series
resistance is relatively more important than shelf life, a number of other
salts can be employed, including KSCN.
The urethane crosslinked elastomers of the polyalkylene oxides can be
produced in a variety of forms depending on the nature of the starting
materials used. A highly crosslinked material is desirable for the
electrolyte coating to be applied to the cathode, as described below, in
order to provide sufficient strength to maintain electrical separation of
the anode and cathode foils. Such a hard elastomeric solid polymer
electrolyte suitable for use as an aluminium capacitor cathode coating is
produced by dissolving NaBF.sub.4 in a mixture of 100 g of
poly(ethyleneglycol) triol (600 daltons), 66 g of the poly(ethyleneglycol)
triol (1000 daltons) (denoted as structure I),
##STR2##
(where in this case R'=H) and then crosslinking with 40 g of hexamethylene
diisocyanate. The mixture is cured either by maintaining at approximately
60.degree.0 C. for 12 hours or by adding a catalyst.
The salt content typically lies in the region of R=10 to 50 where the
concentration unit R is calculated by dividing the number of moles of
oxyalkylene oxygens (or sulphurs in the case of polymers containing
thioalkylene repeat units) by the number of moles of salt in the mixture.
These elastomeric materials also may be prepared in the absence of a
dissolved salt.
This mixture can be easily coated onto the surface of aluminium cathode
foils prior to curing. Once cured, the electrolyte is then found to be
permanently bound to the cathode material. This has a number of
advantages:
i) the electrolyte cannot flow off the foil,
ii) the elastomer provides stress relief to the foil and thus the
foil/electrolyte composite has a higher strength than the foil alone. This
improves the handling of the foil and enables thinner cathode foils to be
employed, resulting in both higher volumetric efficiency and lower weight.
iii) the cathode composite can be prepared separately from the capacitor
rolling step during production.
Softer, and more conductive elastomeric materials suitable for penetration
and operation inside the porous structure of highly etched aluminium
anodes can be prepared by increasing the ratio of poly(ethyleneglycol)
(600 daltons) to the poly(ethyleneglycol) triol in the above mixture and
adjusting the amount of hexamethylene diisocyanate accordingly or by
increasing the glycol and triol chain length, to the extent possible such
that the rubber material cannot crystallize, or by using a mixture of
triol and monool as follows. A mixture of NaBF.sub.4, 137 g of
poly(ethyleneglycol) triol (1000 daltons), 100 g of methoxy
poly(ethyleneglycol) (550 daltons) and 50 g of hexamethylene diisocyanate
is reacted either at elevated temperatures or on the addition of a
catalyst. The salt content typically lies in the region of R=10 to 50. The
product is a clear soft elastomeric material having high conductivity and
low degradative reaction on the aluminium anode.
These elastomeric materials can be prepared either in the absence of a
dissolved salt or if a salt is desired it is added to the mixture of
poly(alkyleneoxides) and dissolved therein prior to addition of the
diisocyanate.
This mixture can be impregnated into the porous structure of highly etched
aluminium anode foils prior to curing. Once cured the electrolyte is then
found to be permanently bound to the anode material. This has a number of
advantages:
i) the electrolyte cannot flow off the anode foil,
ii) the elastomer provides stress relief to the anode foil and thus the
foil/electrolyte composite has a higher strength than the foil alone.
iii) the anode composite can be prepared separately from the capacitor
rolling step during production.
EXAMPLE 7
Referring to FIG. 12, a piece of aluminium cathode foil 16 having a length
of 22 cm and a width of 6.5 cm is coated with a mixture containing 44%
poly(ethyleneglycol) (600 daltons), 29% poly(ethyleneglycol triol) (1000
daltons), 7% NaBF.sub.4 and 20% hexamethylenediisocyanate. Before
application, the salt is dissolved in the glycols and this mixture dried
at 60.degree. C. for 24 hours. On addition of the
hexamethylenediisocyanate, a catalyst such as Thorcat may also be added to
accelerate the curing of the polyurathane at room temperature. After
coating, the foil is cured and dried over silica gel at 60.degree. C.
under vacuum. The resultant coating is a clear tough continuous film of
polyurethane rubber. The coating thickness obtained on the aluminum is
typically 10-20 .mu.m. In a low humidity environment, a piece of aluminium
anode foil 11 having a length of 10 cm, a width of 5.3 cm and having
secured to one end a connector 80 in the form of a conductive tab 4 mm
wide by 8 cm long is then impregnated with a mixture containing 33%
methoxypoly(ethyleneglycol) (550 daltons), 44% poly(ethyleneglycol triol)
(1000 daltons), 7% NaBF.sub.4 and 16% hexamethylenediisocyanate. Before
impregnation the salt is dissolved in the glycols and this mixture dried
at 60.degree. C. for 24 hours. On addition of the
hexamethylenediioscyanate, a catalyst such as Thorcat may also be added to
accelerate the curing of the polyurethane at room temperature. Before the
polyurethane has cured, the liquid impregnated anode 11 is enfolded within
the coated cathode 16 as illustrated in FIG. 12 and the excess liquid
polymer is expelled by rolling. A connector 82, also in the form of a
conductive tab is secured to an uncoated surface of cathode foil 16. Once
cured, the capacitor is formed to full rated surge voltage over a period
of 24 hours, aged at 60.degree. C. for 24 hours and then re-formed.
EXAMPLE 8
In this example the procedure and chemical components are as in Example 7
except that the cathode coating mixture does not contain the salt
component. This produces a cathode coating which, once cured, is less
susceptible to uptake of moisture and therefore has less tendency to
become tacky. Such a composition is suitable where wound rolls of foil are
required not to have high surface adhesion.
EXAMPLE 9
Aluminium cathode foil in wound roll form 6.5 cm wide is coated with a
mixture containing 44 parts poly(ethyleneglycol) (600 daltons), 29 parts
poly(ethyleneglycol triol) (1000 daltons) and 20 parts
hexamethylenedisocyanate. Before application the mixture is dried at
60.degree. C. for 24 hours. The coating mixture is sprayed or rolled onto
both sides of the continuous ribbon of foil and cured by passing the
ribbon through a curing oven maintained at 60.degree. C. After curing, the
foil is wound onto a second drum. The resultant coating is a clear, tough,
continuous film of polyurethane rubber. The coating thickness obtained on
the aluminum is typically 10-20 .mu.m.
In a low humidity environment, a ribbon aluminium anode foil 5.3 cm wide
and having secured at regular intervals a connector tab 4 mm wide by 8 cm
long is then impregnated with a mixture containing 33%
methoxypoly(ethyleneglycol) (550 daltons), 44% poly(ethyleneglycol triol)
(1000 daltons), 7% NaBF.sub.4 and 16% hexamethylenediisocyanate. Before
impregnation the salt is dissolved in the glycols and this mixture dried
at 60.degree. C. for 24 hours. The ribbon is impregnated by passing it
slowly through a bath of the electrolyte mixture. Before the polyurathane
has cured the liquid impregnated anode is compound with a ribbon of coated
cathode foil onto a former 4 mm in diameter. When the required length of
ribbon has been wound onto the former the ribbon is cut, the wound
capacitor removed and winding is restarted on a new former. The excess
polymer liquid is expelled by a pressure roller acting on the former. The
wound capacitors are cured by placing in an oven at 70.degree. C. for 24
hours. Once cured the capacitor is formed to full rated surge voltage over
a period of 24 hours, aged at 60.degree. C. for 24 hours and reformed to
full rated surge voltage.
EXAMPLE 10
A double anode wound capacitor is produced as in Example 9 by passing two
anode ribbons continuously through the impregnating bath and cowinding
both anodes and a single cathode foil onto the former. Such a procedure
produces a further reduction in capacitor volume per unit energy.
EXAMPLE 11
A capacitor having optimum low electrical series resistance is produced
using the method of Example 7 wherein the salt is replaced by KSCN. A
piece of aluminium cathode foil 22 cm long by 6.5 cm wide is coated with a
mixture containing 44 parts poly(ethyleneglycol) (600 daltons), 29 parts
poly(ethyleneglycol triol) (1000 daltons), 6.2 parts KSCN and 20 parts
hexamethylenediisocyanate. Before application, the salt is dissolved in
the glycols and this mixture is dried at 60.degree. C. for 24 hours. On
addition of the hexamethylenediisocyanate a catalyst such as Thorcat may
also be added to accelerate the curing of the polyurethane at room
temperature. After coating, the foil is cured and dried over silica gel at
60.degree. C. under vacuum. The resultant coating is a clear tough
continuous film of polyurethane rubber. The coating thickness obtained on
the aluminum is typically 10-20 .mu.m. In a low humidity environment, a
piece of aluminium anode foil 10 cm long by 5.3 cm wide and having secured
to one end a connector tab 4 mm wide by 8 cm long is then impregnated with
a mixture containing 33 parts methoxypoly(ethyleneglycol) (550 daltons),
44 parts poly(ethyleneglycol triol) (1000 daltons), 6.2 parts KSCN and 16
parts hexamethylenediisocyanate. Before impregnation the salt is dissolved
in the glycols and this mixture dried at 60.degree. C. for 24 hours. On
addition of the hexamethylenediisocyanate a catalyst such as Thorcat may
also be added to accelerate the curing of the polyurathane at room
temperature. Before the polyurethane has cured, the liquid impregnated
anode is enfolded within the coated cathode as indicated in FIG. 12 and
the excess liquid polymer expelled by rolling. Once cured, the capacitor
is formed to full rated surge voltage over a period of 24 hours, aged at
60.degree. C. for 24 hours and then re-formed.
EXAMPLE 12
A rolled capacitor having optimum, low electrical series resistance is
produced as in the method of Example 9 wherein the salt is replaced by
KSCN.
EXAMPLE 13
In this example the procedure is as in example 7 with the chemical
compositions of the cathode and anode as follows: The cathode mix contains
7.8% poly(ethyleneglycol monomethyl ether) (550 daltons), 78.4% Daltocel
34AI (ICI) (5000 daltons), 8.6% NaBF.sub.4 and 5.2%
hexamethylenediisocyanate. The anode mixture contains 33%
poly(ethyleneglycol monomethly ether) (550 daltons), 33.7% Daltocel 34AI
(ICI) (5000 Daltons), 51.9% tetraethyleneglycol dimethyl ether, 8.9%
NaBF.sub.4 and 2.2% hexamethylenediisocyanate.
Referring to FIG. 13, in accordance with yet another variation of the
invention, an anode foil 90 and a cathode foil 92 may be assembled with an
ultrathin spacer 94 used to maintain separation between the foils. The
spacer must be less than 25 .mu. thick and be of an open weave
construction having, for example, openings occupying more than twenty per
cent by volume, and in a preferred embodiment, approximately forty percent
by volume of the spacer material. An example of such a material is
microporous isotactic polypropylene available from Hoechst Celenese
Corporation and called Celgard.RTM. 2500.
The spacer 94 is cowound with the foils 90 and 92. The resulting assembly
is impregnated with an electrolyte according to the invention. The polymer
is then cured. Suitable electrical conductors (leads) are attached to the
foils, and the entire assembly is dip coated with an appropriate
insulating material which is cured to become a comformal capacitor housing
with leads extending therefrom, as is well known in the art. Advantages of
this construction are ease of manufacture and low electrical series
resistance.
EXAMPLE 14
In this example, the procedure and chemical compositions are as in Example
13 except that the cathode coating is omitted and instead a microporous
spacer (Celgard.RTM. 2500) is inserted between the cathode anode foils.
Although the invention has been described with reference to particular
embodiments and examples, it is to be understood that these embodiments
and examples are merely illustrative of the principles of the invention.
Numerous modifications may be made therein without departing from the
spirit and scope of the invention.
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